1. Field of the Invention
The present invention relates to a bar code reading apparatus for the sensing of a bar code along a scanning direction, comprising a light source to produce a transmitted light beam and a diaphragm to restrict the cross-section of the transmitted light beam, wherein the diaphragm has a diaphragm aperture, the boundary of which has four adjoining sections, of which two sections lie opposite to one another and extend substantially transverse to the scanning direction (transverse sections), and of which two sections lie opposite to one another and extend substantially along the scanning direction (longitudinal sections).
2. Description of the Prior Art
Such bar code reading apparatuses serve for the scanning of bar codes. The scanning direction corresponds to the direction of relative movement between the transmitted light beam and the bar code. A relative movement of this kind can, for example, be achieved when the bar code reading apparatus has a deflection unit, by which the transmitted light beam is periodically deflected along the scanning direction at a stationary bar code, or in which a bar code is moved along the scanning direction through a stationary transmitted light beam.
The transmitted light beam, which illuminates the bar code, appears as a light bead on the latter and must have a sufficiently small extent for the correct identification of the bars of the bar code. In many applications, the largest possible depth of focus is required for the bar code reading apparatus, i.e. the light bead should retain a sufficiently small extent along the largest possible reading range in front of and behind the bar code. For this purpose, in addition to the use of image forming optical systems, the use of diaphragms with diaphragm apertures is known, which restrict the cross-section of the transmitted light beam and permit the boundary of the aperture to be subdivided in the initially named manner with respect to the scanning direction into two oppositely disposed transverse sections and two oppositely disposed longitudinal sections.
The limitation of the extent of the light bead by such diaphragms with correspondingly small openings is, however, restricted as a result of diffraction effects; the diffraction of the transmitted light beam at the boundary of the diaphragm opening means that the intensity distribution of the light bead can differ considerably from the original uniform distribution. These diffraction effects at the bar code, however, disturb the correct identification of the bars or their width in a disadvantageous manner. As a result of these effects the diaphragm aperture and thus the cross-section of the transmitted light beam can therefore not be reduced as desired, and the depth of focus of the known bar code reading apparatuses is restricted to an undesired degree.
It is an object of the invention to provide a bar code reading apparatus of the initially named kind, which has an improved depth of focus.
This object is achieved, on the one hand, in that for the compensation of diffraction effects of the transmitted light beam the boundary of the diaphragm opening has at least one compensatory projection at at least one transverse section which projects into the diaphragm aperture as an intermediate extremity with respect to this transverse section in the direction towards the oppositely disposed transverse section.
Since the compensatory projection of the invention points in the direction towards the respective oppositely disposed transverse section of the diaphragm opening, it extends at least approximately parallel to the scanning direction.
The design of the compensatory projection as an intermediate extremity is defined in that the boundary of the diaphragm aperture at the relevant transverse section is set back on both sides of the compensatory projection relative to the latter. Mathematically considered the compensatory projection with its extent along the scanning direction thus forms a maximum with respect to the relevant transverse section. The maximum can in this sense represent a local or an absolute maximum with respect to the two ends of the relevant transverse section or with respect to possible further compensatory projections.
The boundary of the relevant diaphragm aperture can, for example, have a substantially rectangular basic shape, with either the two shorter or the two longer sides of the rectangle extending transversely to the scanning direction and thus, in accordance with the definition, forming the transverse section provided with a compensatory projection. In similar manner the boundary of the diaphragm aperture can also have an oval or round basic shape.
The compensatory projection of the invention brings about in desired manner an at least partial smudging or superimposition of those diffraction effects which are caused by the stopping down of the transmitted light beam at the boundary of the diaphragm aperture and which produce fluctuations of the intensity distribution of the resulting light bead at a bar code. A desired compensation arises in that the compensatory projection itself causes diffraction effects, which at least partly counteract the undesired diffraction effects and smudge or mutually smear the latter.
The compensatory projection thus causes an approximately continuous and uniform transition of the intensity from the center of the transmitted light beam to its edge region. In doing so, it is not essential that all diffraction effects are cancelled, but rather than the negative effects are at least substantially reduced, so that in advantageous manner a more pronounced bounding and bundling of the transmitted light beam can be achieved. In this way a greater depth of focus is achieved with the bar code reading apparatus of the invention.
The diaphragm of the invention proves to be particularly advantageous if the cross-section in accordance with the invention is intended to blank out a part of the transmitted light beam which has a relatively high or indeed similar intensity in comparison to the center of the cross-section of the transmitted light beam, which ultimately leads to pronounced diffraction effects.
This case can, for example, occur if a transmitted light beam with elliptic cross-section is to be restricted by means of a diaphragm aperture of basic rectangular shape in such a way that the transverse sections of the boundary of the diaphragm aperture blank out a significantly greater part of the elliptic cross-section of the transmitted light beam than the longitudinal sections.
The fact that the compensatory projection is provided at at least one transverse section of the diaphragm aperture, i.e. at a section which stands transversely to and thus substantially perpendicular to the scanning direction, yields the following special advantage: Through such a compensatory projection the diffraction maxima and minima of the light bead at the bar code, which are caused by the boundary of the diaphragm aperture, can in particular be reduced. These diffraction maxima and minima otherwise appear as an intensity pattern extending substantially parallel to the direction in which the bar code bars extend and thus particularly greatly impair a correct identification of the bar code. Such diffraction maxima and diffraction minima are broken through by the diffraction effects caused by the compensatory projection.
A further advantage of the diaphragm of the invention lies in the fact that it can be manufactured in simple manner. The openings of customary diaphragms are, for example, produced by laser cutting, etching, punching or drilling. In this connection the provision of one or more compensatory projections in the boundary of the diaphragm aperture represents no significant extra cost or effort or only a slight additional cost and effort.
In preferred embodiments of the compensatory projection of the invention, the latter is formed as a tooth or as a tongue, in which, for example, two straight or curved boundaries form the outline of the compensatory projection. These two boundaries can include an acute or an obtuse angle with one another. The compensatory projection can also be formed as a uniformly roundly shaped tongue, or as a rectangular tongue. Furthermore, it is possible to endow the outline of the compensatory projection with a stochastic pattern which intentionally produces chaotic diffraction patterns for the desired purpose of compensation of diffraction effects.
Finally, it is also possible to provide the compensatory projection additionally with lateral cutouts or projections in order to further enhance the desired effect of the mutual compensation of the diffraction effects that are produced.
The boundary of the diaphragm aperture can have a compensatory projection at its two transverse sections, so that the desired compensation of diffraction effects takes place both in the selected direction of scanning as well as opposite thereto. Furthermore it is preferred if the relevant transverse section or the two transverse sections have a plurality of compensatory projections. Moreover, one or more compensatory projections of the named kind can also be formed at a longitudinal section or at both longitudinal sections of the boundary of the diaphragm aperture.
Through a plurality of compensatory projections the compensation of diffraction effects can be particularly effectively produced by their mutual mixing and blurring. These several compensatory projections preferably form a stochastic arrangement, i.e. a non-periodic and in particular intentionally chaotic arrangement along the relevant transverse section. In this way the occurrence of periodic diffraction or interference patterns can be avoided. A periodic arrangement of a plurality of compensatory projections can also bring about the desired compensation of the diffraction effects.
The object of the invention is solved for a bar code reading apparatus of the initially named kind, amongst other things, also in that a filter is arranged at at least one transverse section or longitudinal section of the boundary of the diaphragm aperture for the compensation of diffraction effects of the transmitted light beam and brings about a continuous or a step-wise transition for the transmission of the transmitted light beam through the diaphragm aperture from substantially 100% at the center of the diaphragm aperture to 0% outside of the boundary of the diaphragm aperture.
Thus, this possibility of solution also aims at compensating for the diffraction effects which occur at the boundary of the diaphragm aperture by damping the light intensity.
The distribution of the degree of transmission of the filter can, for example, be a continuous distribution, in particular a linear or a Gaussian curve distribution. The degree of transmission can, however, also have a discontinuous, in particular a step-likes, distribution, for example in order to intentionally damp individual diffraction maxima, which would otherwise express themselves as an intensity maximum in a light bead at a bar code. Alternatively, the filter can also be formed as least regionally as a gray filter, so that it has a substantially constant degree of transmission in these regions. A filter of the explained kind is preferably provided at the two transverse sections of the boundary of the diaphragm aperture. It is, however, also possible for each section of the boundary to have such a filter. The filter can in particular be formed as a round filter, which is arranged along the entire boundary of the diaphragm aperture.
The invention, moreover, extends to a diaphragm with a diaphragm aperture, by which the cross-section of a light beam is to be restricted, and the boundary of which has two longitudinal sections and two transverse sections, insofar as this diaphragm has features of a diaphragm such as is described in connection with the bar code reading apparatus of the invention. A diaphragm of this kind can also be used in diverse manner in apparatuses other than the bar code reading apparatus of the invention.